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Gram-Scale Synthesis of Submicrometer-Long Polythiophene Wires in Mesoporous Silica Matrices.

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Communications
Molecular Polymer Wires
Polyanilin and polyacryl fibers were first synthesized
inside MCM-41 by oxidation and free-radical initiation of the
Gram-Scale Synthesis of Submicrometer-Long
corresponding monomers.[8–9] Contactless microwave conducPolythiophene Wires in Mesoporous Silica
tivity measurements displayed a high conductivity of the
Matrices**
encapsulated polymer as compared with the bulk polymer.
Since then the polymerization of various monomers, such as
styrene, alkyne, ethylene, and methylmethacrylate, have been
Guangtao Li, Sheshanath Bhosale, Tianyu Wang,
reported within MCM-41.[3–13] The described polymerization
Yang Zhang, Hesun Zhu, and Jrgen-Hinrich Fuhrhop*
processes occur in two steps, 1) loading of the monomer into
preformed silica channels by chemical vapor deposition
(CVD) or freeze-thaw cycles, and 2) polymerization by
The mesoporous silica MCM-41 is characterized by ordered
irradiation or oxidation.
hexagonal arrays of uniform channels. The pore diameter can
It is conceivable that the structure-directing agents for the
be controlled in the range of 2–10 nm by the synthetic
silicate, for example, cetyltrimethylammonium bromide,
procedures and the surface area is about 1000 m2g 1.[1–2] These
could also bear polymerizable units. Aida and Tajima applied
attributes make the material attractive not only as sorbent,
this approach on polyacetylene, but obtained only a few
catalytic support, and separation membrane, but also as ideal
macroscopic rods, with diameters between 15 and 25 mm.[13]
host for rational nanomanufacturing,[3–7] especially as syn[8–13]
thetic scaffolds for controlled polymerization.
We assumed that flexible thiophene units with pentyl or
undecyl ammonium side chains may form more fluid, densely
packed template monomers and could be directly
polymerized in situ in a controllable fashion to linear
macromolecular fibers. Using this strategy (Figure 1),
we report herein the fabrication of submicrometer
long conjugated polythiophene molecular wires of
molecular thinness in gram-scale. The surfactants,
which function both as template and monomer,
trimethyl-(11-thiophen-3-yl-undecyl)
ammonium
bromide (2) and the pentyl homologue 1 were
synthesized by standard methods.[14] Mesostructured
silicates were synthesized following a slightly modified procedure reported by Ozin et al. for the
preparation of mesoporous silica with functional
organic groups within the channels.[15] In a typical
synthesis, 1.0 g of tetraethylorthosilicate (TEOS) was
added to an aqueous solution of NH4OH containing
monomer 2 under stirring. The molar ratio of
1.0 Si:114 H2O:8.0 NH4OH (28 %):0.12 thiophene 2
was thus established in the reaction mixture. After
60 min stirring at room temperature, the solution was
left standing at 90 8C for 5 h in a closed polyethylene
bottle. A white powder was collected by filtration,
washed thoroughly with millipore water, and dried in
air. The yield was about 1 g. We assume that all of the
thiophene 2 was entrapped in the material, because
Figure 1. Schematic representation of the fabrication and organization of conjugated polythiophene molecular wires with mesoporous silica MCM-41.
the filtrate seemed not to contain a detergent. The
homologous monomer 1 was less useful as a template,
presumably,
because of its shorter chain length, it does not
[*] Prof. Dr. J.-H. Fuhrhop, Dr. G. Li, S. Bhosale, Dr. T. Wang
form
micelles.
Freie Universit.t Berlin
Figure 2 A shows the powder X-ray diffraction (PXRD)
Institut f0r Organische Chemie
Takustrasse 3, 14195-Berlin (Germany)
pattern of the resultant material 3. Three resolved peaks
Fax: (+ 49) 30-8385-5589
could be indexed as (100), (110), and (200) and are typical of
E-mail: fuhrhop@chemie.fu-berlin.de
the reflections of a well-ordered hexagonal mesoporous solid.
Dr. G. Li, Dr. Y. Zhang, Prof. Dr. H. Zhu
The position of the intense (100) peak reflects a d spacing of
Department of Chemistry
approximately 3.6 nm, which is clearly larger than that of
Tsinghua University
mesostructured silica prepared with tetradecyltrimethylamBeijing (China)
monium bromide (which has the same chain length as 2) as
[**] Financial support by the Deutsche Forschungsgemeinschaft
template. Complementary to the XRD data, transmission
(SFB 448 “Mesoscopic Systems”), the Fonds der Deutschen
electron microscopy (TEM) reveals micropores with a
Chemischen Industrie, and by the FNK of the Free University is
hexagonal symmetry throughout the sample with a center to
gratefully acknowledged.
3818
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200351158
Angew. Chem. Int. Ed. 2003, 42, 3818 –3821
Angewandte
Chemie
Figure 2. A) Powder X-ray diffraction pattern and B) TEM image of
synthesized mesoporous silica using 2 as structure-directing agent.
center distance between the hexagon centers of 3.4–3.8 nm
(Figure 2 B). FT-IR spectra display bands close to 1500 cm 1
(1490, 1479, and 1467 cm 1), which are characteristic for the
stretching of thiophene ring.
The polymerization of densely packed thiophene monomer in MCM-41 was carried out in 10 mL of a CH2Cl2
dispersion containing 2 g of synthesized silica and 8 g of
FeCl3.[16, 17] The reaction mixture was stirred at room temperature for 2 days and the solvent removed. The residue was
then dispersed in 10 mL of 0.3 m HF in water. An orange solid
4 was formed, collected by filtration, washed thoroughly with
water, and dried in air. We obtained 240 mg of the polymer
from 2 g TEOS and 1.0 g from 10 g TEOS.
The polymer was very soluble in water and produced a
broad absorption band in the UV/Vis spectrum with a
maximum at 465 nm as well as a fluorescence band centered
at 610 nm, which both indicate the formation of a conjugated
polymer. The same polymer in the MCM-matrix showed an
absorption maximum at 505 nm and an emission at 640 nm.
The red-shift clearly indicates the confinement effect of the
nanoscale silicate channel on the effective conjugation length
of the polymer chain formed. This effect was also found for
other semiconducting polymer chromophores which have
been encapsulated into the hexagonally arrayed channels of
mesoporous silica glass by a post-loading approach.[18, 19] The
IR-spectrum showed essentially the same thiophene bands as
the monomer (1490, 1485, and 1467 cm 1).
TEM provided direct visualization of the polythiophene
wires formed. Figure 3 A shows TEM images of a sample
Angew. Chem. Int. Ed. 2003, 42, 3818 –3821
Figure 3. TEM images of A) the conjugated molecular wires from the
polymerization of 2 and B) the organized wires upon the addition of
PF6 ions.
stained by 1 % phosphotungstate (pH 7). It is clear that
massive, coiled, individual strands are distributed over large
areas. These strands have a tendency to entangle with each
other. Attempts to obtain separated single strands by dilution
and to detect clear-cut endings and thus determine the length
of individual strands failed. The strands have, however, a
uniform diameter of 3 nm (Figure 3 A), which is in excellent
agreement with the width of unidecylammonium thiophene
repeating units in the all-trans conformation in a micellar
double strand (Figure 1). This result suggests that the in situ
polymerization in mesoporous silica produces well-resolved
individual molecular wires. This is quite different from other
studies, where analogous reactions in porous membranes gave
nanorods or nanotubes with submicrometer diameters.[20–22]
From the images, it was estimated that the average length of
the conjugated chains is at least 100 nm corresponding to
approximately 330 thiophene units. Attempts to obtain
MALDI-TOF spectra gave no positive results (matrix:
terthiophene).[23] The polymer was free of low-molecularweight components. It was also found that upon the addition
of PF6 to neutralize the ammonium groups the polymer
changed from being in a disordered state into a semiordered
state with chains of up to 150-nm in length (Figure 3 B).
Thin polymer films were spin-coated from aqueous
solutions containing about 10 mg of the polymer per 1 mL
water onto various substrates and electrodes. Figure 4 shows
www.angewandte.org
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3819
Communications
Figure 4. Cyclic voltammograms of spin-coated film of the polymer of
2 on ITO-electrode in 0.1 m Bu4NPF6/CH2Cl2 as a function of scan
rates.
the cyclic voltammograms (CVs) of polymer films in CH2Cl2
with 0.1m Bu4NPF6 using various scan rates. The polymer film
displayed the typical reversible oxidative processes of conjugated polythiophene at a potential of 1.0 V (vs. Ag/AgCl).
The current is linearly proportional to the scan rate
(Figure 4), which indicates that the electroactive polymer
was in contact with the electrode (electrode supported). The
system was unusually stable; even after 20 CV cycles no
decrease of the signal intensity was observed. Using interdigitated electrodes for the measurement we observed
sigmoidal curves indicating a maximum conductivity of
2 S cm 1 at room temperature and in the presence of air.
This conductivity is comparable to that reported for polyalkylthiophene.[23]
Figure 5 illustrates the optoelectrochemical spectra
obtained with synthesized polymer on an ITO-coated glass
plate. The neutral-state polymer has its absorption maximum
at 420 nm. Upon oxidation the maximum absorption is shifted
to 780 nm which is accompanied by a color change from
reddish violet to dark blue. This characteristic p–p* transition
of conjugated polythiophenes is completely reversible in this
polymer. The fluorescence can be effectively quenched by
negative charged anthraquinone molecules (Figure 5 B).
In conclusion, our method provides photoluminescent
silica composite and also aligned conjugated 100-nm polymer
chains on a gram scale. Since cetyltrimethylamonium bromide
can been replaced by other detergent matrices in the synthesis
of MCM-41,[24–26] one may also equip the thiophene bolaam-
3820
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 5. A) UV/Vis absorption spectra of spin-coated film of the polymer of 2 on ITO-plate: a) neutral state and b) oxidized state; B) fluorescence quenching of aqueous solution of synthesized polythiophene
wires using 9,10-anthraquinone-2,6-disulphonic acid sodium salt (5 mL
each of a 0.1 m solutions).
phiphiles with different head groups (e.g. poly(ethylene
glycol) or phosphonate). This would make conducting wires
accessible, which can be combined in assemblies.
Received: February 11, 2003
Revised: May 2, 2003 [Z51158]
.
Keywords: mesoporous materials · molecular wires ·
polythiophene · silicates · template synthesis
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